It's possible that when you logged into the sync servers, they thought your new system was the old one and so updated its data with the new system. To avoid something like this, each computer system and profile needs to have its own Device Name. Type '''about:preferences#sync'''<enter> in the address box. Look for '''Device Name''' and change it how you want it. -------------- Restore Bookmark Backups Firefox automatically makes backups of the bookmarks. These are stored in the '''bookmarkbackups''' folder. Open the '''Bookmarks Manager; ''' Press the '''Alt''' or '''F10''' key to bring up the toolbar, and select '''Bookmarks. ''' Hot key is '''<Control> ''(Mac=<Command>)'' <Shift> B. ''' Firefox 84+ '''<Control> ''(Mac=<Command>)'' <Shift> O. ''' Once the window is open, at the top of the page, press the button labeled '''Import and Backup. ''' Select '''Restore. ''' From the list, pick the file you want and click it. The backups are stored in a '''jsonlz4 file''' named something like this; bookmarks-2016-06-29_1424_enD1GTH ya0JmicnYWg4V4g==.jsonlz4 In this example; 2016-06-29 = date created, 1424 = number of entries. https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them https://support.mozilla.org/en-US/kb/recover-lost-or-missing-bookmarks https://support.mozilla.org/kb/forum-response-recover-bookmarks-after-update